It frequently happens, in the optical field, that signals which are trigonometrical functions of a magnitude to be measured are produced or have to be processed.
In particular, as shown in the article by A. J. Rogers: "Optical methods for measurement of voltage and current on power systems/Optics and Laser Technology/December 1977/pages 273 to 283", or the article by A. M. Smith: "Polarization and magnetooptic properties of single-mode optical fiber/Applied optics, vol. 17, No. 1/Jan. 1 1978/pages 52 to 56", or again the article by A. Papp and H. Harms: "Magnetooptical current transformer/Applied optics/15 November 1980/Vol. 19, No. 22/pages 3729 to 3734", measurements of current by Faraday effect lead to the production of trigonometric signals which are proportional to the sine and cosine of twice the angle of rotation of the plane of polarization of a light beam.
As is known, two out-of-phase trigonometric functions, such as sine and cosine, can define an angle only within a single turn. It is therefore necessary, when an angle is capable of changing through more than 360.degree., to provide means for counting the number of turns effected by this angle in one direction or the other.
An example of a known apparatus permitting the measurement of an angle in a range greater than 360.degree. is given by German Pat. No. DE 30 06 840.
However, this apparatus is adapted only for the measurement of the phase difference between a reference signal and a measurement signal, and does not correspond to the introduction developed hereinabove.